Certain embodiments of the invention relate to communication of signals in optical transmission networks. More specifically, certain embodiments of the invention relate to a method and system for pattern-independent phase adjustment in a clock and data recovery (CDR) circuit.
Synchronous optical network (SONET) and synchronous digital hierarchy (SDH) include a set of related standards that define synchronous data transmission over fiber optic transmission networks. SONET defines a United States (US) version of the standard which is published by the American National Standards Institute (ANSI). However, SDH defines the international version of the standard published by the International Telecommunications Union (ITU). Together, the SONET and SDH standards ensure interconnectivity and interoperability between local and international optical networks and also among conventional and legacy systems.
SONET has a nominal base transmission line rate of 51.84 Mbps, known as optical carrier level one (1) OC-1 or synchronous transport signal level one (1) or STS-1. SONET provides a set of multiples of the base rate known as OC-1, up to a current maximum line rate of 9.953 gigabits per second (Gbps). Notwithstanding, actual line rates in excess of 20 Gbps have be attained. The table below shows the various data rates for SONET and SDH as defined by their respective standards and specifications.
OpticalElectricalLine RatePayload RateOverheadLevelLevel(Mbps)(Mbps)Rate (Mbps)SDHOC-1STS-151.84050.1121.728—OC-3STS-3155.520150.3365.184STM-1OC-9STS-9466.560451.00815.552STM-3OC-12STS-12622.080601.34420.736STM-4OC-18STS-18933.120902.01631.104STM-6OC-24STS-241244.1601202.68841.472STM-8OC-36STS-361866.2401804.03262.208STM-12OC-48STS-482488.3202405.37682.944STM-16OC-96STS-964976.6404810.752165.888STM-32OC-192STS-1929953.2809621.504331.776STM-64
The use of SONET in transport networks is fairly widespread and SONET is considered the foundation for the physical layer of broadband ISDN (BISDN). Transport protocols such as the well known asynchronous transfer mode (ATM) runs as a layer on top of SONET as well as other transport technologies. ATM utilizes a cell-based structure consisting of short fixed length packets called cells. These cells may be adapted to facilitate fast efficient packet switching and routing. Accordingly, a received payload may be multiplexed into these cells and quickly routed to its destination. This may be particularly useful in broadband networks that carry diverse content such as voice, video, data and images, some of which may include time critical data and subject to strict latency requirements and/or bandwidth constraints.
Transport networks using SONET and/or SDH may provide more powerful networking capabilities than existing asynchronous systems. Since synchronous transmission networks such as SONET and SDH utilize a highly stable reference clock signal, there is no need to locally align clock signals or provide clock synchronization in order to recover data. Accordingly, it is possible to recover much lower data rates such as digital system one (1) (DS-1), without having to demultiplex an entire bit stream, as would be required for conventional asynchronous transports networks.
SONET and/or SDH may also provide interconnectivity between various network vendor products by providing standardized physical layer interfaces. These standardized interfaces may define parameters such as an optical line rate, tolerable power levels, pulse width, light wave length, and various encoding and decoding algorithms. The standards also provide definitions for framing, including frame format and structure, data overheads and payload mapping. Information in the data overhead may facilitate various functions such as operations, administration, maintenance and provisioning (OAM&P).
While synchronous network systems are suitable for point-to-point communication application, SONET and SDH may be adapted to support point-to-point as well as point-to-multipoint arrangements such as hub configuration. In a hub configuration, for example, a hub may be configured to function as an intermediary for traffic which may be distributed to various network components or entities commonly referred to as spokes for the hub. In this regard, the hub may unify communication between network components and entities connected thereto, thereby eliminating inefficient communication between individual network components and entities. The hub may also reduce requirements for back-to-back multiplexing and help to realize the benefits of traffic grooming.
Traffic grooming may include tasks such as network traffic consolidation or segregation of network traffic which may provide more efficient usage of transmission facilities and bandwidth. The consolidation of traffic may include combining traffic from various locations or sources into one central transport facility. Segregation of network traffic received from various locations may include separating network traffic into its various constituent components and/or various logical and physical criteria. Some communication systems use techniques such as backhauling to reduce expenses associated with repeated multiplexing and demultiplexing. However, grooming may be used to eliminate inefficient methodologies such as backhauling. Although grooming may be done with asynchronous system, it may require various expensive internal and infrastructure changes. However, synchronous networks such as SONET and SDH may be more conducive and adaptable to grooming since traffic data may be segregated at either an STS-1 level or a virtual tributary (VT) level and dispatched to various appropriate system components and/or entities for processing.
In SONET networks, for example, grooming may also be provide segregation of services, which may include any one or more of voice, video, and data. For example, at an interconnection point, an incoming SONET line may contain different types of traffic such as switched, video, voice and/or data. In this case, a SONET network may be conveniently adapted to segregate the switched and non-switched traffic. SONET also includes various options that may be configured to facilitate integrated network OAM&P by providing connectivity between a single maintenance point and various network components or entities. In this regard, a single connection may be configured to reach all network elements within a given architecture, thereby eliminating a need for separate links which may be required for maintaining each and every network component or entity. Particularly, SONET provides overhead data that directly permits OAM&P activities such as remote provisioning. Remote provisioning may provide centralized maintenance and reduced maintenance cost. Since SONET provides substantial overhead information, this information may provide enhanced monitoring and maintenance, along with more efficient diagnostic capabilities which may significantly shorten downtime.
In digital transmission systems, clock signals are critical to system operation since they may be used to keep a constant bit rate and to demarcate the various logic levels in a data stream. Traditionally, transmission systems have been asynchronous with each network component providing its own local clock signal. Since the clocks are asynchronous, transitions of the signals do not necessarily occur at the same nominal rate and large variations may occur in the clock rate, resulting in variable bit rate data signals. In a synchronous system such as SONET, the average frequency of all the clocks in the system will be same or very nearly the same. Accordingly, the frequency of the clocks will be synchronous or plesiochronous. Since every clock in the system may be traced back to a highly stable reference clock signal, a base rate or the STS-1 rate will remain at a nominal 51.84 Mbps allowing many synchronous STS-1 signals to be stacked together without any bit stuffing. Low speed synchronous virtual tributaries (VT) may also be interleaved to create much higher transmission data rates using SONET and SDH. At low speed, for example, DS-1's may be transported by synchronous VT's at a constant rate of approximately 1.728 Mbps.
Additionally, synchronous multiplexing may be done in multiple stages to achieve one of a plurality of desired data rate. Accordingly, a signal such as a synchronous DS-1 may be multiplexed and additional bits such as dummy bit may be added to account for variations which may occur in each individual data stream. Each individual DS-1 data stream may be combined with each other to form a DS-2 stream. This process may be repeated and each individual DS-1 data stream may be further combined with each other to form a DS-3 stream.
Notwithstanding the advantages provided by SONET and SDH, synchronization of information during various stages of processing and transport may affect the performance of SONET and SDH optical transmission systems. In CDR of for example, high speed bi-level synchronous signals, a clock signal having a frequency which may be equivalent to the bit rate, may be extracted from the bit stream. In this regard, the extracted clock signal may be and aligned with data in the bit stream in order to extract the data by sampling. However, an optimal sampling point or required phase alignment of any given synchronous signal may vary depending various conditions, including but not limited, to noise and interference. In various applications such as SONET, it may be crucial to have the recovered clock lock to the a desired position within a data eye of sampled portions of the data in the bit stream. This may be referred to as phase adjustment. In such applications, it may be preferable that a locking position remain constant in the face of a changing data pattern density. Furthermore, performance of any voltage controlled oscillator should not be affected by the phase adjustment.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.